The present invention relates to a method for machining the periphery of a work such as a thyristor silicon wafer, and to an apparatus using this method.
A technique for obliquely machining the periphery of a wafer of a thyristor is known as one of various methods for improving withstanding voltage of a large-scale semiconductor rectifying element such as a high-power thyristor. Such oblique machining is performed in the following manner. Referring to FIG. 1, a cup-shaped grinding wheel 1 and a silicon wafer 3 mounted on a base plate 2 are prepared. The wheel 1 and the base plate 2 are each rotated, such that the outer periphery of the wafer 3 abuts against the rotating wheel 1. Then, the periphery of the wafer 3 is machined to provide a trapezoidconical silicon wafer.
The sequence for such machining generally consists of 4 steps: (1) the rotating wafer 3 is brought close (in the direction indicated by arrow M1) to the similarly rotating wheel 1 by fast feeding; (2) the wafer 3 is moved by slow feeding (in the direction indicated by arrow M1) so that the wheel 1 gradually cuts into the periphery of the wafer 3; (3) after completion of machining to a predetermined extent, feeding of the wafer 3 is interrupted, and it is kept in this position (this step is called "spark out"); (4) after the spark out step, the wafer 3 is returned by fast feeding to the origin position in the direction indicated by arrow M2.
In order to perform the above sequence, the position of the wafer 3 relative to the wheel 1 must be set for each of the 4 steps. Such positioning may be performed by hardware such as photosensors or microswitches. More specifically, the position of the wafer 3 or of the base plate 2 on which it is carried is detected by a microswitch or the like either before or after each step so as to determine the start or end timing of the step. However, such positioning using hardware significantly degrades the actual machining efficiency. This may be explained as follows. Every time the size of the wafer 3 or the base plate 2 changes, the distance through which the wafer 3 must be fed also changes. Therefore, every time a change in the size of the wafer 3 or the base plate 2 is made, high-precision adjustment of the position of the microswitch (positioning sensor) must be performed.
A chuck of a tool which holds a work (e.g., a silicon wafer mounted on the base plate) is inclined with respect to the machining surface through, for example, 60.degree.. For this reason, it is difficult to precisely determine the machining start and end positions of the work. This means that precise positioning of the positioning sensor is time-consuming. This difficulty or imprecision in positioning the positioning sensor results in significant error or a variation in the amount of machining executed by the machining surface of the wheel. In addition, the machining surface of the wheel is subject to wear. For this reason, even if a positioning sensor is once positioned with high precision in accordance with the type and size of a work, variations in the machining amount due to the wearing down of the wheel over time cannot be prevented. In order to avoid such variations, the positioning sensor must be precisely repositioned as the wheel is worn.